In a hip replacement coping with a hip osteoarthritis, parts for forming a prosthetic joint are substituted for a caput femoris and an acetabuli of a pelvis, respectively. An operation for the hip osteoarthritis includes removing partially the caput femoris and the acetabuli, and then implanting the parts substituting for the removed portions thereof. Such an operation, which has an influence on a range of movement of a hip joint after completion of the operation, should be performed in an accurate manner.
According to a conventional operation, medical physicians utilize a photograph of an X-ray photography or a fluoroscopy thereof to provide an illustration of a bone, confirm a positional relationship of the bone, and then determine a site of a body to which operation instruments such as an impactor are to be applied and a way of operation based on a body trunk of a patient, which can be obtained through a visual measurement by the medical physicians, by their experiences and intuition, thus advancing the operation.
Even if the experienced operators may acquire an interrelationship between the bone and the operation instruments with a high degree of accuracy to manipulate appropriately the operation instruments in a manner as described above, there is however a high risk that the body of the patient may unexpectedly move. In case where the bone deviates from its original ideal position as set, the operation based on the visual measurement may not cope with even a slight deviation in an operational condition in which an existence of an operation cloth usually disables the operators from directly observing the entire hip of the patient. This makes it not possible to ensure an accuracy of the operation, thus causing problems that a range of movement of the hip joint after completion of the operation may become narrower or a bone dislocation may easily occur.
In view of these problems, there has recently been proposed an operation navigation system in which, three-dimensional spatial data of a bone including a site on which an operation is to be performed are previously prepared from images obtained through a CT scan or an MRI prior to the operation, infrared markers provided on the surface of the body of the patient in the vicinity of the site on which the operation is to be performed and on an operation instrument and a related infrared positional measurement device are used to acquire the positions of the infrared markers during the operation to obtain the spatial positions the bone and the operation instrument, and the thus obtained spatial positions and the direction of the bone are compared with the three-dimensional spatial data of the bone to determine accurate position and direction of the site such as a joint on which the operation is to be performed, and then the operator is navigated with an accurate information on a manipulating direction of the operation instrument relative to the site on which the operation is to be performed, through a laser pointer or a display unit.
For example, Japanese Patent Provisional Publication No. 2002-85421 and Japanese Patent Provisional Publication No. 2005-30911 describe such a conventional operation assisting system.
The above-described conventional operation navigation system gives instructions on a position toward which the operation instrument is moved and a direction along which the operation instrument is directed on the display unit to provide a way, which enables the operation to be performed accurately irrespective of movement of the site on which the operation is to be performed.    Patent Document 1: Japanese Patent Provisional Publication No. 2002-85421    Patent Document 2: Japanese Patent Provisional Publication No. 2005-30911